1. Technical Field
Embodiments described herein relate to an optical fiber temperature distribution measuring device using backward Raman scattering light.
2. Related Art
Among distribution measuring devices using an optical fiber as a sensor, there is a temperature thermometer which measures a temperature distribution along an optical fiber. This technique utilizes backward scattering light that is generated in an optical fiber.
Among various type of backward scattering light such as Rayleigh scattering light, Brillouin scattering, and Raman scattering light, backward Raman scattering light which has high temperature dependence is used for temperature measurements. A measurement is performed by wavelength-dividing backward Raman scattering light. Backward Raman scattering light has two types, that is, anti-Stokes light AS having a shorter wavelength than incident light and Stokes light ST having a longer wavelength than incident light.
Optical fiber temperature distribution measuring devices measure anti-Stokes light intensity Ias and Stokes light intensity Ist, calculate a temperature from their ratio, and display a temperature distribution along the optical fiber. Such optical fiber temperature distribution measuring devices are used in such fields of temperature management of plant facilities, research and study relating to disaster prevention, and air-conditioning of big constructions.
FIG. 8 is a block diagram showing the configuration of an example basic optical fiber temperature distribution measuring device. As shown in FIG. 8, a light source 1 is connected to an input port of an optical demultiplexer 2 and an optical fiber 3 is connected to an input/output port of the optical demultiplexer 2. A photoelectric converter (hereinafter referred to as an O/E converter) 4st is connected to one output port of the optical demultiplexer 2 and an O/E converter 4as is connected to the other output port the optical demultiplexer 2.
An output terminal of the O/E converter 4st is connected to a arithmetic controller 7 via an amplifier 5st and an A/D converter 6st. An output terminal of the O/E converter 4as is connected to the arithmetic controller 7 via an amplifier 5as and an A/D converter 6as. The arithmetic controller 7 is connected to the light source 1 via a pulse generator 8.
The light source 1, which is a laser diode, for example, emits pulse light in synchronism with a timing signal that is supplied from the arithmetic controller 7 via the pulse generator 8. The optical demultiplexer 2 receives, at its input port, the pulse light emitted from the light source 1, and provides the received pulse light to the optical fiber 3 from its input/output port. The optical demultiplexer 2 receives, at its input port, backward Raman scattering light that is generated in the optical fiber 3 and wavelength-divides the backward Raman scattering light into Stokes light and anti-Stokes light. The optical fiber 3 receives, at its input port, the pulse light from the optical demultiplexer 2, and provides the backward Raman scattering light to the optical demultiplexer 2 from its input port.
The O/E converters 4st and 4as are photodiodes, for example. The Stokes light that is output from the one output port of the optical demultiplexer 2 is provided to the O/E converter 4st, and the anti-Stokes light that is output from the other output port of the optical demultiplexer 2 is provided to the O/E converter 4as. The O/E converters 4st and 4as are configured to generate electrical signals corresponding to the received light beams, respectively.
The amplifiers 5st and 5as amplify the electrical signals that are output from the O/E converters 4st and 4as, respectively. The A/D converters 6st and 6as convert signals that are output from the amplifiers 5st and 5as into digital signals, respectively.
The arithmetic controller 7 calculates a temperature based on the digital signals that are output from the A/D converters 6st and 6as, that is, from an intensity ratio between the two components (Stokes light and anti-Stokes light) of the backward scattering light, and displays, on a display unit (not shown), a temperature distribution along the optical fiber 3 based on a resulting time series of temperatures. A relationship between the intensity ratio and the temperature is stored in the arithmetic controller 7 in advance in the form of a table or a formula. The arithmetic controller 7 provides the timing signal to the light source 1 and controls the output timing of light pulses that are output from the light source 1.
Next, the principle of temperature distribution measurement will be now described. Since the light speed in the optical fiber 3 is known, a time function representing the signal intensity of each of Stokes light and anti-Stokes light with a light-emitting time point of the light source 1 as a reference can be converted into a function of the distance along the optical fiber as measured from the light source 1, that is, a distance distribution in which the horizontal axis represents the distance and the vertical axis is the light intensity of Stokes light or anti-Stokes light generated at each position in the optical fiber 3.
On the other hand, each of the anti-Stokes light intensity Ias and the Stokes light intensity Ist and their ratio Ias/Ist depend on the temperature of the optical fiber 3. Therefore, the temperature at a position of generation of Raman scattering light can be obtained if an intensity ratio Ias/Ist becomes known. Since the intensity ratio Ias/Ist is a function Ias(x)/Ist(x) of the distance x, a temperature distribution T(x) along the optical fiber 3 can be obtained from the intensity ratio Ias(x)/Ist(x).
FIG. 9 is a block diagram of an optical fiber temperature distribution measuring device in a related art. Components having the same components in FIG. 8 are given the same reference symbols as the latter.
As shown in FIG. 9, a temperature reference unit 9 having a rolled-up optical fiber of several tens of meters is provided between the optical demultiplexer 2 and the optical fiber 3 (connected to the optical fiber 3 via a connector 13.) The temperature reference unit 9 is provided with a thermometer 10 which has a platinum resistance thermometer sensor, for example, and configured to measure an actual temperature. An output signal of the thermometer 10 is provided to the arithmetic controller 7. A reference thermometer 11 which has a platinum resistance thermometer sensor, for example, and configured to measure an actual temperature is also provided in the vicinity of the optical fiber 3 which is used as a temperature sensor.
With the above configuration, when the temperature of the optical fiber 3 is T (K), the intensity ratio Ias/Ist between anti-Stokes light and Stokes light is given by the following Equation (1):
                                          I            as                                I            st                          =                              G            as                    ×                                    (                                                                    ω                    0                                    +                                      ω                    r                                                                                        ω                    0                                    -                                      ω                    r                                                              )                        4                    ×                      exp            ⁡                          (                              -                                                      h                    ⁢                                                                                  ⁢                                          ω                      r                                                                            2                    ⁢                    π                    ⁢                                                                                  ⁢                    kT                                                              )                                                          (        1        )            where
Gas: ratio of an anti-Stokes light gain to a Stokes light gain;
ω0: angular frequency of an optical signal;
ωr: Raman shift angular frequency of the temperature reference unit 9;
h: Planck constant (6.626×10−34 J·s); and
k: Boltzmann constant (1.38×10−23 J·K−1).
Although the parameter
                              L          n                =                              G            as                    ×                                    (                                                                    ω                    0                                    +                                      ω                    r                                                                                        ω                    0                                    -                                      ω                    r                                                              )                        4                                              (        2        )            
Ln is unknown in an actual system, but Ln can be calculated using temperature data of the temperature sensor 10 which is provided in the temperature reference unit 9.
Letting T0 and G0(T0) represent the temperature measured by the temperature sensor 10 and the corresponding intensity ratio Ias/Ist, respectively. As shown below, Equation (3) is obtained from Equations (1) and (2).
                              L          n                =                                            G              0                        ⁡                          (                              T                0                            )                                ×                      exp            ⁡                          (                                                h                  ⁢                                                                          ⁢                                      ω                    r                                                                    2                  ⁢                  π                  ⁢                                                                          ⁢                                      kT                    0                                                              )                                                          (        3        )            
Using this value, an equation for calculating a temperature T from an intensity ratio Ias/Ist between anti-Stokes light AS and Stokes light ST is given by:
                                                        T              =                            ⁢                                                                    h                    ⁢                                                                                  ⁢                                          ω                      r                                                                            2                    ⁢                    π                    ⁢                                                                                  ⁢                    k                                                  ×                                  1                                                                                    -                        log                                            ⁢                                                                        I                          as                                                                          I                          st                                                                                      +                                          log                      ⁢                                                                                          ⁢                                              L                        n                                                                                                                                                                    =                            ⁢                                                                    h                    ⁢                                                                                  ⁢                                          ω                      r                                                                            2                    ⁢                    π                    ⁢                                                                                  ⁢                    k                                                  ×                                                      1                                                                                            -                          log                                                ⁢                                                                              I                            as                                                                                I                            st                                                                                              +                                              log                        ⁢                                                                                                  ⁢                                                                              G                            0                                                    ⁡                                                      (                                                          T                              0                                                        )                                                                                              +                                                                        h                          ⁢                                                                                                          ⁢                                                      ω                            r                                                                                                    2                          ⁢                          π                          ⁢                                                                                                          ⁢                                                      kT                            0                                                                                                                                .                                                                                        (        4        )            
Actually, an error is caused by a loss occurring at the connecting portion between the device main body and the optical fiber 3 and the difference between a Raman shift angular frequency of the optical fiber 3 and a Raman shift angular frequency used in the calculation. Therefore, a true temperature is measured by the reference thermometer 11 which is disposed in the vicinity of the optical fiber 3, and the Raman shift angular frequency ωr used for the temperature calculation is adjusted finely and a temperature T calculated according to Equation (4) is corrected using a coefficient and an offset.
For example, a coefficient correction and an offset correction are used in the following manner:Tr=A×T+C where
Tr: corrected temperature (K);
T: temperature (K) before correction;
A: correction coefficient; and
C: correction offset.
To eliminate an error that is caused by a loss occurring at the connecting portion and the difference between the Raman shift angular frequency of the optical fiber used in the temperature reference unit 9 and that of the optical fiber 3 used as the sensor, JP-A-2008-249515, for example, proposes a method in which a temperature reference unit to provide a reference for temperature calculation outside the device main body, that is, on the path of the optical fiber for measurement.
However, in the related-art configuration shown in FIG. 9, to perform fitting while adjusting the Raman shift angular frequency finely, it is necessary to repeatedly measure a temperature while finely adjusting the Raman shift angular frequency used for the calculation. Thus, the calibration takes long time.
Even after the fine adjustment of the Raman shift angular frequency, a temperature measurement error occurs due to the difference between the Raman shift angular frequency of the optical fiber used in the temperature reference unit 9 and Raman shift angular frequency of the optical fiber 3 used as the temperature sensor.
Furthermore, a correction that is performed using a coefficient and an offset is a linear correction whereas the relationship between the Raman scattering intensity and the temperature is nonlinear. This causes large errors at temperatures that are much different from the temperature used for the correction.
Still further, where the temperature reference unit for providing a reference for temperature calculation outside the device main body, that is, on the path of the optical fiber for measurement, the total configuration becomes complex. The temperature reference unit needs to be constructed at an installation site of the optical fiber for measurement, resulting in a problem that temperature correction work is complicated.